WiFi isn't a guaranteed medium for constant high speeds. 350mbps sounds about right for an IPad as many mobile devices support around the 433mbps link speed.
Added to that is wireless congestion from neighbours WiFi and the overhead and your chances of 500mbps over WiFi are not that great, let alone 433.
Your WiFi broadcasts on 2 bands, 5ghz and 2.4ghz:
5ghz has greater speed but limited range compared to 2.4ghz which has greater range but lesser speed.
Your connection could be dropping from 5ghz to 2.4ghz on your IPad which may explain the speed drop to 30. It will drop to 2.4ghz as you move further away from the router.
As you pointed out resetting helps, because it will intiallity connect at 5ghz but the iPad may see 2.4ghz as a more strong and stable signal so might drop back down to it.
There might be a setting on the IPad to force it to stay on 5ghz, but I am not sure as I do not own one to test. If such a setting exists, it should be somewhere under the WiFi settings.
